We have a 1920s house with a ground floor bathroom. The bathroom is entered through what was once the old back door to the house and there is a step down of approx 130mm into it. The concrete slab floor in there is currently tiled, but I’m fairly sure not insulated as it’s freezing cold most of the time.

As part of the bathroom renovations, I’d like to insulate the floor, ( perhaps adding electric underfloor heating) and raising the floor level at the same time to match that of the adjacent room. If it makes any difference, I’d also like to insulate the walls and ceiling at the same time. The existing ceilings are at the same level in both rooms so height is not an issue.

A few Google searches say that the best method for this (typically when converting a garage into a usable room) is to lay rigid form insulation (e.g. Celotex etc.) directly onto the slab, then a damp proof course, then an 18mm ply/chipbord floating floor over the top.

So far so good, but would a floor like this take the weight of bathroom fittings and could I tile onto it? Surely it would be too flexible/delicate? Should I lay joists and insulate between them instead? Any suggestions?

A few Google searches say that the best method for this (typically when converting a garage into a usable room) is to lay rigid form insulation (e.g. Celotex etc.) directly onto the slab, then a damp proof course, then an 18mm ply/chipbord floating floor over the top.

My concrete workshop is insulated like this:

Celotex G4100 on the floor by brf, on Flickr

Floor insulation complete by brf, on Flickr

The 18mm Ply spreads the load very evenly, I drop 100kg + bar bells on it and had no problems so far, so I doubt bathroom fittings will be a problem.

i would put some joists down an insulate in between, then put down 25mm ply, fixings at minimum 200mm centres. any movement under tiles will result in water ingress and tiles lifting. use noggins between joists to stop them twisting as they dry out

As above, lay the insulation boards (100mm ?) straight into the slab, and floating floor on top (18/22mm Ply)

Have done a fair few like that and there is no problem whatsoever if its all done properly.

If you float a floor over insulation you WON”T be able to tile it. they’ll be too much movement in it

I’d opt for a DPM, then thinner 50mm insulation layer, then the electric cables for UFH ( you can get a specific cable for this rather than the under tile type) then a 50mm screed over the lot. you can then tile this or whatever finish you like.

My kitchen is done in the above manor and is good after 5 years so far, the floor warms up quickly and stays warm.

Blazin (Kitchen and Bathroom installer)

I would personally put joists and noggins so that you have less movement and something sturdy to screw the toilet into, but that might be a bit ‘belt-and-braces’. The feet for a shower tray will just screw into ply. Tiling on top of ply will also be fine as long as you keep the floor relatively dry (grout is porous and ply will swell quickly if it gets damp and won’t return to its original shape). If you are likely to spill a lot of water (i.e. if you have kids / dogs) you might want to research tanking underneath the tiles.

I’d check with your local building control on whether you need regulations for this work.

wwaswas – Member
I’d check with your local building control on whether you need regulations for this work.

I would not.

do exactly as blazing saddles suggested, I’ve done several this way.

Thanks for all the replies, everyone. The floating floor would be simple, but clearly tiling then becomes an issue and I can’t think how else I’d want to cover a bathroom floor. So that leaves the joist/timbers option as suggested by blazing saddles and others.

Questions: Would the dpc be the first thing to go down onto the existing floor, then the joists/timbers on top? Would I lay the timbers so they are resting or supported by the existing floor, or would I use hangers from the wall? If hangers, is there a height they would need to be off the floor and would this need ventilation? ( which could be a problem). I’m also not sure about laying a 50mm screed – is that fairly simple to do?

Thanks for everyone’s help so far.

Would I lay the timbers so they are resting or supported by the existing floor, or would I use hangers from the wall? If hangers, is there a height they would need to be off the floor and would this need ventilation? ( which could be a problem)

Little bump…can anyone answer those two questions for me?